Superconductivity was first observed by the Dutch physicist H. K. Onnes in 1911 during his investigations of the electrical conductivities of metals at very low temperatures. Onnes observed that as purified mercury is cooled, its electrical resistivity vanishes abruptly at a temperature of 4.16K. Above this temperature, the electrical resistivity is small but finite and measurable; alternatively, when the temperature is reduced below 4.16K, the electrical resistivity is so small that it is effectively zero. This distinct temperature at which the transition and loss of effective electrical resistivity occurs has been termed the critical temperature or "T.sub.c ". Onnes believed he had discovered a new physical state of matter at temperatures below the critical temperature and coined the term "superconducting state" for the observed phenomenon at temperatures below the critical temperature (T.sub.c) and the term "normal state" for the electrical properties observed at temperatures above the critical temperature. Onnes also found that the superconducting transition is reversible and that the superconducting material recovered its normal, electrical resistivity at the critical temperature.
The modern theory of superconductivity is the result of the research investigations by Bardeen, Cooper, and Schrieffer [Phys. Rev 106:162 (1957)]. Their proposal, conventionally known as the "BCS theory", has now gained universal acceptance because it has proved capable of explaining most of the observed phenomena relating to superconductivity. Their principles employ a quantum mechanical treatment of the superconductive phenomenon; and their theory has been employed to explain the various observable effects such as zero electrical resistance, the Meissner effect, and the like. Since the BCS theory is so steeped in quantum mechanics, the reader is directed to published texts in the scientific literature for a complete description and explanation. These include: M. A. Omar, Elementary Solid State Physics: Principles and Applications, Addison-Wesley Publishing Company, 1975, pages 496-527; M. Tinkham, Introduction to Superconductivity, McGraw-Hill Co., 1975.
Superconductivity has been found not to be a rare phenomenon. It is exhibited by a substantial number of atomic elements, metallic alloys, and most recently, refractory oxide ceramics. For many years, the highest known critical temperature was only 23K. There has, accordingly, been intense interest and research investigations into finding superconductive materials with much higher critical temperatures, most desirably those which hopefully would approach room temperature (20.degree. C.). Until relatively recently, efforts to achieve this goal have met with complete failure. Beginning about 1986, however, polycrystalline scintered ceramic pellets of yttrium-barium-copper oxide and mixtures of thalium, strontium barium, bismuth, and copper oxygen have been found to demonstrate relatively high critical temperatures (T.sub.c) and superconductivity at temperatures up to 120K [Bednorz, J. G. and K. A. Muller, Z. Phys. B64:189 (1986); Wu et al., Phys. Rev. Lett, 58:905 (1987); and Chu et al., Phys. Rev. Lett. 60:941 (1988)]. These compounds are now conventionally termed high transition temperature or "high T.sub.c " superconductors.
Since about 1986, interest in superconductive materials as potential replacements for conventionally known metal in wiring and microcircuitry has risen appreciably; and the search for ever-higher T.sub.c superconductors in various formats is presently an area of intense exploration. Mersly representative of these continuing research investigations and recently reported developments are the following publications: Experimental Techniques in Condensed Matter Physics at Low Temperatures, (R. C. Richardson and E. N. Smith, editors), Addison Wesley Inc., 1988, pages 118-123; TG. K. White, Experimental Techniques in Low-Temperature Physics, Oxford University Press, 1959, pages 295-298; Advances in Superconductivity, Proceedings of the 1st International Symposium on Superconductivity, August 1988, Nagoya, Japan; Yeh et al., Phys. Rev. B36:2414 (1987); Morelli et al., Phys. Rev. B36:3917 (1987); Chaudhari et al., Phys. Rev. B36:8903(1987); Tachikawa et al., Proc. IEEE 77:1124 (1989); Tabuchi et al., Appl. Phys. Lett. 53:606 (1989); Sacchi et al., Appl. Phys. Lett. 5:3:1111 ( 1988); Abell et al., Physica C162-164:1265 (1989); Bailey et al., Physica C167:133 (1990); Xiao et al., Phys. Rev. B36:2382 (1987); Matsuda et al., Mat. Res. Soc. Symp. proc. 99:695 (1988); Witanachchi et al., J. Mater. Res. 5:717 (1990); Superconductive Industry, Winter, 1989, page 31; Engineer's Guide to High-Temperature Superconductivity, Wiley & Sons, Inc. 1989; and D. Newman, Superconductive Industry 3:16 (1990).
One recurring kind of problem for superconductors generally has been the electrical joining and union of superconductive materials, particularly the juncture of high T.sub.c superconductors or "FITSC", to each other and to other electrically conductive materials in the normal state at temperatures between 70K and 300K and to conventional superconductive materials which have a transition temperature below 30K. By definition, electrically conductive materials in the normal state include both the normal conductors such as gold, silver, cooper, and iron; and the semi-conductors such as carbon, silicon, gray tin, and germanium; as well as their respective mixtures with indium, gallium, antimony, and arsenic. It is also difficult to make effective low resistance juncture and electrical union with the atomic elements and alloys most frequently used in practical superconducting applications. These typically are the conventionally known superconductors Nb, NbTi, and NbSn; and these frequently serve as materials to make superconducting motors, generators, and magnets which operate at liquid helium temperature (4.5K).
Traditionally, solders--a general term for alloys useful for joining metals together by the process of soldering--have been used directly as an intermediate to join superconductors to themselves, to semi-conductors, and to normal conductors. The principal types of solder conventionally known are: soft solders such as lead-tin alloys; and brazing solders such as alloys of copper and zinc and sometimes silver. Representative of conventionally known solders and soldering techniques are U.S. Pat. No. 3,600,144 describing a low melting point brazing alloy; U.S. Pat. No. 4,050,956 describing a method of chemically bonding metals to refractory oxide ceramics; U.S. Pat. No. 4,580,714 disclosing a hard solder alloy comprising copper, titanium, aluminum, and vanadium; U.S. Pat. No. 4,582,240 revealing a method for intermetallic diffusion bonding of piezo-electric components; U.S. Pat. No. 4,621,761 identifying a brazing process for forming strong joints between metals and ceramics while limiting the brazing temperature to not more than 750.degree. C.; and U.S. Pat. No. 4,631,099 describing a method for adhesion of oxide type ceramics and copper or a copper alloy. Unfortunately, solders alone and the conventionally known soldering techniques have proven inadequate for junctures of high T.sub.c superconductors.
For this reason, many investigators have attempted to generate specialized techniques for lowering the resistance of electrical contacts between two high T.sub.c superconductive materials or between high T.sub.c superconductors and a metal. Representative examples of these techniques include: vapor deposition of silver followed by annealing bulk sintered samples of yttrium-barium-copper oxide at temperatures up to 500.degree. C. for an hour [Superconductor NewsMay-June, 1988, page 5]; the use of laser energy to deposit a thin film of superconductive yttrium-barium-copper oxide directly onto a silicon substrate [Superconductor News, May-June, 1988, page 1]; electrolytic depositing of gold onto superconducting particles [U.S. Pat. No. 4,971,944]; sputter depositing a layer of silver on a yttrium-barium-copper oxide surface [Ekin et al., Appl. Phys. Lett. 52 (1988)]; U.S. Pat. No. 4,963,523]; deposition of silver or gold on a superconductive material [Van der Mass et al., Nature 328:603 (1987)]; thermal evaporation of silver on a yttrium-barium-copper oxide surface [Tzeng et al., Appl. Phys. Lett. 52 (1988)]; the use of silver paste on superconductive materials [Munakata. et al., Jap J. Appl. Phys. 26: (1987)]; the pressing of platinum against YBCO [Grader, G. S., App. Phys. Lett. 51: (1987)]; and spark bonding [Lye et al., Jap. J. App. Phys. 27: (1988)].
In addition, see also Ekin et al., App. Phys. Lett. 52:331 (1988); Elkin et al., App. Phys. Lett. 52:1819 (1988); Suzuki et al., Appl Phys. Lett. 54:666 (1989); Jin et al., Appl. Phys. Lett. 54:2605 (1989); and Katz et al., J. Appl. Phys. 65:1792 (1989). These publications describe the formation and use of different types of contact pads as current and voltage leads including indium contacts; ultrasonically soldered bonds; silver paste which is first baked and then soldered into the superconductor; sputtering; metal spraying; and silver vapor deposition.
Recently, a major improvement in preparing electrical contacts of minimal resistance for superconductors has occurred using a metallic particle diffusion method. This particle diffusion procedure uses noble metals such as silver in fragmented or powder form which are applied externally to the superconductor as a series of individual surface coatings; each coating is then sintered individually to its softening point; and then each is cooled such that it becomes embedded internally within the grains of the superconductive material. A final external coating provides a solidified contact of markedly reduced electrical resistance.
Another kind of continuing and concomitant problem of superconductors is the limited critical current density able to be conveyed by superconductors generally and the major deterioration of the limited critical current capacity upon the application or in the presence of a magnetic field. The critical current density ("J.sub.c ") of a superconductive material is defined as the maximal load of current density ("J.sub.c ") able to be conveyed or carried by a superconductor before it becomes a normal conductor. "J.sub.c " is a function of temperature. For commercially available HTSC bulk materials generally, only very low loads of critical current density in the range of 400 amps per cm.sup.2 to about 700 amps per cm.sup.2 in a zero magnetic field are typical. Moreover, even these very limited load values of critical current capacity for superconductors become markedly reduced and greatly deteriorated when a magnetic field is applied to the superconductor.
Note that if in a perfect superconductor the energy of the shielding current exceeds the difference between the normal and superconducting free energy, the superconductor becomes normal. In HTSC materials, the critical current is limited by weak links between grains of superconductive material which are affected to a greater extent than single crystals by the magnetic field [M. Tinkman, Introduction To Superconductivity, McGraw-Hill, 1975].
Note also that the high T.sub.c materials belong to the category of type II superconductors, i.e., some flux is locked into the superconductor. When a superconducting current density, J.sub.c, flows perpendicular to the direction of fluxoids (flux lines)(.PHI.=h/2e), the fluxoids are forced to move along the direction perpendicular to the current flow by the Lorent.sub.3 force (F=J.times..PHI.). This movements of fluxoids results in a finite resistivity caused by energy dissipation due to the pointing vector if the flux moves. The movement of the fluxoids can be due to an external magnetic field. Therefore, the pinning of fluxoids is necessary for the suppression of such resistivity. It is important that this array of fluxoids remains stable, since motion results in dissipation by normal currents induced in the fluxoid cores. Fluxoid stability and critical current density can be increased by the presence of impurities or defects, which tends to stabilize or "pin" the fluxoids; and there are several ways to create pinning centers including radiation defects, inclusion of a non-superconducting phase, and addition of silver. [Avvides etal., Physica C 179 (1991 ); Kes et al., Cyrogenics 29 (1989); Kubo et al., Phys. Rev. B39 (1989); and Yamaguchi et al., J. Appl. Phys. 29 (1990)].
The application of high transition temperature superconductors (HTSC) for practical purposes, has thus been limited by their low critical currents and the deterioration of this critical current on the application of a magnetic field. Although some HTSC materials have been produced which have critical current densities in the order of 10.sup.5 A/cm.sup.2, these can only be made as films or in small laboratory quantities; and cannot be produced in moderate quantities because of the slow rate and the cumbersome and unreliable process by which they are produced [Izuni et al, J. Mater Res. 7: (1992); Jin et al., Phys. Rev. B37: (1988); Monot et al., J. Mater. Res. 7: (1992); and Salama et al., Appl. Phys. Lett. 54:(1989)]. The forms of HTSC materials which can be produced in quantity are bulk sintered components which typically are produced from a powder. In general, the; powder is mixed with an organic binder which is then carefully removed. The surface tension of the binder provides the necessary force to bring the powder grains together so that they can be sintered once the binder is eliminated.
The ultimate limitation on J.sub.c is thought to be the weak link behavior which develops between the powder grains during sintering [Gotch et al., J. Appl. Phys. 72:15 (1992)]. This behavior is enhanced, or may be a result of the extremely shod coherence length in these materials. In long lengths of the material, the critical current of the HTSC is also limited by micro-cracks, and the alignment of the grains since their conductivity is anisotropic. To a certain degree, those micro cracks can be filled by the addition of silver powder to the HTSC powder before the binder is added and before sintering [Neal et al., IEEE Transactions, Applied Superconductivity 1:(1991); and Itoh, M. and H. Ishigaki, J. Mater. Res. 6:2272 (1991)]. In addition, the silver may provide pinning centers for quantized magnetic flux lines and thus diminish flux motion, which contributes to resistivity in superconducting materials. The presence of silver leads to the increase of J.sub.c by increasing the resistance to thermal shock and subcritical crack growth via improvements in mechanical properties (such as strength and fracture toughness). Silver powder fills the weak links in the gaps between the grains and forms a conductive path for the current. Also, the J.sub.c is increased because the silver acts as a pinning center/pinning the magnetic; field as noted above. [Singh et al. J. Mater. Res. 8:(1993); Reich, S. and I. Felner, J. Appl. Phys. 67: (1990); and Peters et al., Appl. Phys. Lett. 52:(1988)]. The addition of silver, however, increases the thermal conductivity of the material--which in many applications is an undesirable effect since silver is a metal and has a higher thermal conductivity than HTSC.
Clearly, therefore, there; remains a well recognized need for effective means and methodology by which to meaningfully increase the critical current of superconductors which are electrically functional at temperatures between 100K-4.5K or lower and to avoid the deleterious effects of a magnetic field. If such effective means and methods were available, the electrical junction and union between the circuits and instruments of our everyday world could then be linked and employed in combination with the enhanced critical current capacity of circuits provided by superconductors generally. Such effective means and methodology, however, have as yet not been available in this art.